1. Field of the Invention
The present invention relates to a method for producing a polyoxyalkylene glycol by subjecting a cyclic ether to ring opening polymerization in the presence of a zeolite as a catalyst. Particularly, it relates to a method for producing polytetramethylene ether glycol from tetrahydrofuran. More particularly, it relates to a method for producing polytetramethylene ether glycol having a narrow molecular weight distribution.
Another object of the present invention is to provide metallo-aluminosilicates having a novel structure which is useful as a catalyst for ring opening polymerization of a cyclic ether.
2. Discussion of Background
Polytetramethylene ether glycol (hereinafter referred to simply as "PTMG") is a straight chain polyether glycol having primary hydroxyl groups at both terminals, which is represented by a general formula HO-[(CH.sub.2).sub.4 O].sub.n --H, and it is a moderate molecular weight polymer useful as a starting material for polyurethane elastic fibers for which stretchability and resilience are required. Recently, it is useful also as a starting material for thermoplastic elastomers. For a starting material for such elastic fibers or elastomers, it is common to employ PTMG having a number average molecular weight (Mn) of from about 500 to 3,000. Accordingly, PTMG having a molecular weight within this range is produced in large amount.
There are known some methods which are conventional for producing PTMG. Namely, (1) a method wherein tetrahydrofuran (hereinafter referred to simply as "THF") is polymerized using as a catalyst a protonic acid belonging to a super strong acid, such as fluorosulfonic acid or fuming sulfuric acid, to obtain a polymer with both terminals being esterified with the protonic acid, and then the terminals are converted to hydroxyl groups by hydrolysis, to obtain a PTMG, (2) a method wherein THF is polymerized using as a catalyst a mixture of an acid and acetic anhydride, such as a mixture of perchloric acid and acetic anhydride or a mixture of a super strong acid containing a fluorine atom and acetic anhydride, to obtain a polymer with both terminals being esterified with acetic anhydride and then the ester groups at both terminals are hydrolyzed with an alkali to convert the terminals to hydroxyl groups, to obtain PTMG, (3) a method wherein THF is polymerized in the presence of a carboxylic anhydride using as a catalyst a perfluorosulfonic acid resin made by copolymerization of e.g. tetrafluoroethylene or chlorotrifluoroethylene with a perfluoroalkylvinyl ether containing a sulfonic acid group precursor (a sulfonic acid group-forming group), to obtain a polymer with both terminals being esterified, and then the ester groups at both terminals of the polymer are subjected to alcoholysis in the presence of a catalyst such as calcium oxide in a basic medium, to convert the terminals to hydroxyl groups, to obtain PTMG, (4) a method wherein THF is polymerized using activated clay as a catalyst to obtain PTMG, (5) a method wherein THF is polymerized using as a catalyst a super strong acid prepared by treating zirconia with sulfuric acid to obtain PTMG, and (6) a method wherein THF is polymerized using as a catalyst a heteropolyacid wherein the content of water of crystallization is controlled within a specific range, to obtain PTMG.
However, in the methods (1) and (2), large amounts of the catalyst are required, and the catalyst is decomposed in the hydrolysis step and can not be reused. Further, a large amount of a waste water is formed in the hydrolysis step, and the pollutant must be removed from the water before the water is discharged to a sewer. Furthermore, the catalyst used here is highly corrosive, and an expensive material has to be used for the construction of the apparatus. In the method (3), a very expensive resin is used as the catalyst, such being economically disadvantageous. In the method (4), the activated clay, which is obtained by acid treatment of montmorillonite which is a natural ore of smectites-type, is basically a natural product, and its composition and the amount of impurities are not constant, whereby there has been a problem that the performance is not consistent. In the method (5), the catalyst to be used, is prepared by e.g. a method which comprises impregnating concentrated sulfuric acid to zirconia, followed by evaporation to dryness and then by calcination to support sulfate groups on zirconia. However, the preparation is difficult and dangerous. Further in the presence of water, sulfate groups contained in the catalyst are lost as sulfuric acid, whereby there has been a problem that the catalytic activities tend to deteriorate. In the method (6), a large amount of heteropolyacid is required as the catalyst, and it is required to strictly control the content of water of crystallization in the acid. Further, the catalytic solution comprising heteropolyacid, THF and water, and another solution comprising PTMG, and unreacted THF must be separated by decantation after the polymerization reaction. Thus, this method is inferior in the operation efficiency and the economical aspect, as compared with a case where a solid catalyst is used, wherein the catalyst can easily be separated by filtration from the resulted polymerization reaction mixture.